1. Field of the Invention
The present invention relates to a recordable optical recording medium, in particular, a surface-side reproduction type optical recording medium wherein recording or reproducing is conducted by irradiating light from a front surface side of the optical recording medium.
2. Discussion of Background
An optical recording medium is a recording medium permitting large quantity and high density recording, for which there has been an increase of demand as a medium for a large volume file of computer or for recording dynamic images, in current fashion of multimedia.
The optical recording medium generally comprises a transparent circular disc-like substrate made of a plastic material or the like on which a multilayer including a recording layer is formed. Recording or erasing is carried out by irradiating laser light, and reproducing is carried out by reflection light of the laser light. The optical recording medium is classified into an erasable type capable of rewriting such as a magneto-optical recording medium or a phase-change recording medium and a write-once type capable of writing only once such as CD-R.
In the magneto-optical recording medium, a so-called light intensity modulation recording has mainly been used wherein information is erased by applying a fixed magnetic field, and then, recording is carried out by applying a fixed magnetic field in an opposite direction. However, in recent years, there has been noted a magnetic field modulation system wherein a magnetic field is modulated according to a recording pattern while laser light is irradiated because such system can record correctly information in one revolution (direct over-write) even at a high recording density.
The phase-change recording medium has recently been used widely because it is possible to conduct the direct over-writing by an optical modulation recording system and to conduct reproducing by the using the same optical system as for CD or DVD. Further, a write-once type CD-R is widely spread because it is completely compatible with CD.
As means for improving the recording density of the optical recording medium, a land/groove recording system, wherein recording is conducted to both a groove which has conventionally been used merely as a guide groove and a land as a recording area respectively, has been noted (Symposium of optical memories ""94 Technical Digest p.41 (1994), Jpn, J. Appl. Phys. Vol. 37 p.2144 (1998)).
In such system, the reproduction of signals is obtainable with little crosstalk by using a technique that the width of a land is substantially equal to the width of a groove and the optical depth of the groove is about ⅙ of the laser wavelength, or a technique in combination of the optical depth being about xe2x85x9 of the laser wavelength and independent phase adjustment on the land and the groove at the time of reproducing.
Conventionally, laser for recording or reproducing is irradiated to the recording layer through the substrate. As a technique of bringing an optical head close to the recording layer to conduct recording or reproducing, a so-called near field optical recording has been noted as means for increasing recording density (Appl. Phys. Lett. 68, p. 141 (1996). In this recording method, a head having a solid immersion lens (hereinbelow, referred to as SIL) is used to reduce the spot size of laser beams, whereby the reproducing is possible with marks which is shorter than the limit in the conventional recording technique determined by a laser wavelength (xcex) of a light source (xcx9cxcex/2NA where NA represents a numerical aperture of objective lens), and recording or reproducing can be realized at a super high recording density.
In the near field optical recording, it is necessary to bring the optical head closer to the recording medium (200 nm or less). Accordingly, unlike the conventional magneto-optical recording medium to which laser beams are irradiated to the recording layer through the substrate, a method for irradiating directly laser beams to the recording layer without passing through the substrate, is used (surface-readout type recording). In this case, use of a flying type slider head is proposed in order to bring SIL head closer to the recording layer.
In an attempt of continuous tests for recording and reproducing near field optical recording media by using the above-mentioned SIL head by the inventors of this application, there has been found that when recording is continued for a long time, foreign matters deposit on the SIL head whereby recording and reproducing can not be continued.
Further, since the medium surface of the groove is remote from the optical head by the distance corresponding to the depth of the groove in comparison with the medium surface of the land, there is apt to occur the total reflection of laser beams beyond the critical angle on the groove in conducting the recording or the reproducing in the land/groove recording system, hence, light convergence properties become poor, with the result that the recording/reproducing properties of the groove are deteriorated in comparison with the recording/reproducing properties of the land.
It is an object of the present invention to provide an optical recording medium suitable for near field optical recording, which exhibits high reliability in continuous tests for recording and good recording/reproducing characteristics.
It is another object of the present invention to provide an optical recording medium exhibiting sufficient recording/reproducing characteristics even in a land/groove recording system.
In considering the above-mentioned circumstances, the inventors have made extensive studies on the structure of a recording medium in consideration that the power density is extremely high at a lower surface of the SIL lens and the medium surface, and accordingly, if a substance or a defect which absorbs laser, exists on the medium surface or in an optical path around the lower surface of the SIL head, temperature will rise abruptly to result a foreign matter, and have completed the present invention by providing a transparent protective resin layer having a refractive index np of more than NA on the recording layer and determining the diameter of laser beams on the medium surface to be 1.5 xcexcm or more.
In accordance with the present invention, there is provided a surface-side reproduction type optical recording medium in which at least a recording layer and a transparent protective resin layer having a refractive index np of more than the NA are formed on the substrate, and which carries out recording or reproducing with laser beams from an optical head having a numerical aperture NA of more than 1 wherein the optical head is floated from the surface of the medium at a height of xcex/4 or less where xcex is the wavelength of the laser beams, the surface-side reproduction type optical recording medium being characterized in that the diameter of the laser beams on the medium surface is from 1.5 xcexcm to 30 xcexcm. In a magneto-optical recording medium or a phase-change recording medium in the present invention, it is preferable to form a dielectric layer having a refractive index which is at least 0.4 larger than the NA on the recording layer, and to form the above-mentioned transparent protective resin layer on the dielectric layer.
Further, in accordance with the present invention, there is provided a surface-side reproduction type optical recording medium for carrying out recording or reproducing with laser beams from an optical head having a numerical aperture NA of more than 1 wherein the optical head is floated from the surface of the medium at a height of xcex/4 or less where xcex is the wavelength of the laser beams, the surface-side reproduction type optical recording medium being characterized in that at least a recording layer and a transparent protective resin layer having a refractive index np of more than the NA are formed on the substrate, and the transparent protective resin layer includes ultra-fine particles of inorganic material having a diameter of 0.05 xcexcm or less.
Further, in accordance with the present invention, there is provided a surface-side reproduction type optical recording medium for carrying out recording or reproducing with laser beams from an optical head having a numerical aperture NA of more than 1 wherein the optical head is floated from the surface of the medium at a height of xcex/4 or less where xcex is the wavelength of the laser beams, the surface-side reproduction type optical recording medium being characterized in that at least a recording layer, a dielectric layer and a transparent protective resin layer having a refractive index nd of more than the NA are formed on the substrate in this order, and the refractive index nd of the dielectric layer is 2.4 or more.
Further, in accordance with the present invention, there is provided a surface-side reproduction type optical recording medium for carrying out recording or reproducing with laser beams from an optical head having a numerical aperture NA of more than 1 wherein the optical head is floated from the surface of the medium at a height of xcex/4 or less where xcex is the wavelength of the laser beams, the surface-side reproduction type optical recording medium being characterized in that a land and a groove are formed in a substrate surface; at least a recording layer and a transparent protective resin layer having a refractive index np of more than the NA are formed on the substrate, and the step formed between the land and the groove in the surface of the transparent protective resin layer is xcex/30 or less.